Break at_ifawithnet() into two variants:

• at_ifawithnet(), which acquires an locks it needs and returns an at_ifaddr reference.
• at_ifawithnet_locked(), which relies on the caller locking at_ifaddr_list, and returns a pointer rather than a reference.

Update various consumers to prefer one or the other, including ether
and fddi output, to properly release at_ifaddr references.

Rework at_control() to manage locking and references in a manner
identical to in_control().

MFC after: 6 weeks